It was narrated from 3Abdullaah ibn 3Amr ibn al-3Aas (may Allaah be pleased with him) that the Prophet sallAllaahu 3Alayhi wa Sallam said: “The best of du3aa’ is du3aa’ on the day of 3Arafah, and the best that I and the Prophets before me said is:
لا إله إلا الله وحده لا شريك له ، له الملك ، وله الحمد ، وهو على كل شيء قدير‘
Laa ilaaha ill-Allaah wahdahu la shareeka lahu, lahul-mulk wa lahul-hamd wa huwa 3ala kulli shay’in qadeer’
(There is no god but Allaah alone, with no partner or associate; His is the dominion, to Him be praise, and He has power over all things).” (1)
It was narrated from Talhah ibn 3Ubayd ibn Kurayz in a mursal report: “The best of du3aa’ is du3aa’ on the day of 3Arafah.” (2).
The scholars differed concerning this virtue of du3aa’ on the day of 3Arafah, whether it applies only to those who are in 3Arafah or it includes other places. The more correct view is that it is general and that the virtue is connected to the day, but undoubtedly those who are in 3Arafah have combined the virtue of the place with the virtue of the time.
Al-Baaji (may Allaah have mercy on him) said:
The words “the best of du3aa’ is on the day of 3Arafah” mean the dhikr with the greatest blessing, the greatest reward and that which is most likely to be answered. It may be interpreted as referring to pilgrims only, because the meaning of du3aa’ on the day of 3Arafah is very true in their case, and applies especially to them. But if the day of 3Arafah is regarded in general terms, it is described as such because of the pilgrims being there. And Allaah knows best”. (3)
Excerpt from Islamqa.com